A cruise control system in a vehicle provides automatic speed control to enable the vehicle to maintain constant speed under variable driving conditions without driver's intervention. A conventional cruise control system in an electric vehicle controls an electric motor of the vehicle to request a torque value required to achieve a desired speed.
Under typical driving conditions, torque values required to achieve a desired speed are subject to wide variability with little, if any, long term predictability. Moreover, driving conditions, such as steep uphill grade or heavy vehicle load or the like, may impose limitations on available speed and acceleration.
Higher acceleration or greater speed may be required than the system can accommodate at maximum torque restricted by available motor power supply. In particular, the voltage signal that the motor control needs to produce in order to request the torque required to achieve the desired speed may be greater that the supply voltage. Therefore, the motor would not be able to develop the required torque.
In addition, in a vehicle drive environment, wherein power availability is limited to an on-board supply, it is highly desirable to attain a high torque output capability at minimum power consumption. Motor structural arrangements described in the copending applications contribute to these objectives. As described in those applications, electromagnet core segments may be configured as isolated magnetically permeable structures in an annular ring to provide increased flux concentration. Isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects occurring from interaction with other electromagnet members.
The above-identified co-pending application Ser. No. 10/173,610 describes a control system for a multiphase motor that compensates for variations in individual phase circuit elements. A high degree of precision controllability is obtained with each phase control loop closely matched with its corresponding winding and structure. Successive switched energization of each phase winding is governed by a controller that generates signals in accordance with parameters associated with the respective stator phase components. The phase windings are energized with current of sinusoidal waveform for high efficiency operation. The control system varies the output current to respond to, and accurately track, the torque command input.
The sinusoidal current waveform profile obtained with this commutation strategy can extend battery life through efficient operation. However, in vehicle driving operation there may be a need for torque capability in excess of that available from the most efficient control scheme. Typically, the power supply is rated for a maximum current discharge rate, for example, 10.0 amps. If the cruise control system requests a torque command that correlates to this maximum current draw, then the motor torque output for a sinusoidal current waveform profile is limited, for example, to approximately 54.0 Nm in a motor with a configuration such as described above.
The above-identified copending application Ser. No. 10/386,599 describes a cruise control system including a control circuit for producing a control signal to control an electric motor of the vehicle. The control signal is formed based on a control current required to achieve the desired speed. The system determines a motor control scheme that provides an appropriate waveform profile of the control current for available driving conditions. In particular, the system performs switching between a high-efficiency motor control scheme that provides a substantially sinusoidal waveform profile of the control current for achieving operating efficiency of the motor, and a high-torque motor control scheme that provides a substantially rectangular waveform profile of the control current for achieving high torque. The replacement of the high-efficiency control scheme with the high-torque control scheme results in a higher torque needed when torque obtainable with the high-efficiency control scheme is not sufficient for the cruise control system to maintain a desired speed. However, the motor operating at the high-torque control scheme sacrifices some of the efficiency achievable with the sinusoidal waveform profile.
Accordingly, it would be desirable to maintain operations with a substantially sinusoidal waveform profile as long as the required torque is achievable using a high-efficiency control scheme.
Hence, the need exists for precision adaptive motor control that would extend a range of motor operation at a high-efficiency motor control scheme in a cruise control system having various motor control schemes.